A typical hybrid hydraulic drive system uses a reversible pump/motor to either store power from or add power to a conventional vehicle drive system. The system stores power by pumping hydraulic fluid from a low pressure reservoir into a hydraulic energy storage system. This hydraulic energy storage system typically consists of one or more nitrogen-charged hydraulic accumulators. In many cases the hydraulic energy storage system can also absorb power and reject it into the atmosphere as waste heat energy rather than storing it. Hybrid hydraulic drive systems typically add power to conventional vehicle drive systems by utilizing the hydraulic energy stored in the hydraulic accumulator to drive the reversible pump/motor as a motor.